+1 on bettas needing 5+ gallons in the long term. They may survive in smaller, but don't have the room to perform natural behaviours. Just as an aside, although bubble-nesting is super cool and certainly not a bad thing, it doesn't necessarily mean that a fish is "happy."
In addition to the betta not having enough space, smaller tanks will experience parameter swings much more easily and more intensely, so again, 5 gallons+ (preferably 10+) means that you're not running your tank on a razor's edge.
Absolute no on crayfish with fish - it's not a case of "if" there's problems, but "when", especially with slower fish such as male bettas.
Why not invest in 2x 10 gallon tanks, one for fish and one for crayfish? You could have some really great complementary aquascaping or something, that seems to be quite a trend recently.
For the 2.6 gallon, you could use it for shrimp (if you're confident in keeping water parameters stable) or maybe just as a back up for moving fish/treatments/tank leaks etc. I have an empty 5 gallon for this and it's super useful